In general, a bill identification apparatus, which is one of the embodiments of the paper sheet identification apparatus, is incorporated into a service device such as a game medium rental machine installed in a game hall, a vending machine or a ticket-vending machine installed in a public space, or the like which identifies the validity of a bill inserted from a bill insertion slot by a user and provides various types of products and services in accordance with a value of the bill having been judged as legitimate.
Usually, the authenticity of the bill is identified by a bill identification apparatus installed in a bill traveling route continuously extending from a bill insertion slot. The bill moving inside the bill traveling route is irradiated with light, and a transmitted light and a reflected light therefrom are received by a light receiving sensor, and the received light data is compared with the legitimate data to identify the authenticity of the bill.
Meanwhile, various innovations have been devised for bills in order to prevent counterfeiting thereof. As one of those, a watermark with an uneven portrait is formed by a special technique, or a see-through patterned mark which can be determined as authentic or counterfeit with a tactile sense is formed (hereinafter, watermarks formed on bills or see-through patterning are collectively referred to as “a watermark”). Such a watermark may be utilized as an authenticity identification object area in order to improve the identification accuracy of the authenticity of the bill. In Patent reference 1, for example, a bill discrimination device is disclosed, which discriminates the authenticity of the bill by irradiating an infrared light and a visible light to a watermark and acquiring a transmitted light and a reflected light therefrom.    [Patent Reference 1] Japanese unexamined patent application publication No. 2006-285775